Electrically conductive polymers are the focus of considerable interest as they are possible replacements for metallic conductors or semi-conductors in a wide variety of applications such as in batteries, in photovoltaics, and in electrostatic dissipation and electromagnetic shielding uses.
Known methods of preparing electrically conductive polymers include an oxidative polymerization of aromatic compounds such as pyrrole or aniline.
The oxidative polymerization can be carried out electrochemically or by means of a chemical oxidizing agent. The electrically conductive polymers are prepared in the form of powders, coatings or free-standing films.
It is also known to carry out the oxidative polymerization in the presence of a carrier whereby an electrically conductive polymer composition of the aromatic polymer and the carrier material is produced. The known methods are discussed below in more detail.
It is well known that polypyrrole can be synthesized through electrochemical oxidative polymerization of pyrrole (A. F. Diarz et al., J.C.S., Chem. Comm., 1979, pages 635 and 636). Unfortunately, the mechanical properties of the produced polypyrrole coatings or films are often unsatisfactory unless special care is taken to chose the proper type of solvents or conducting salts such as aromatic sulfonic, disulfonic or oligosulfonic acids disclosed in DE-A-34 21 296 and DE-A-33 18 857.
It is also known that pyrrole can be polymerized chemically, for example with potassium peroxydisulfate, potassium permanganate, sodium perborate, iron(III) chloride or potassium bichromate in the absence or presence of a conducting salt, (K. C. Khulbe and R. S. Mann, Journal of Polymer Science, Vol. 20, pages 1089 et seq., 1982, S. P. Armes, Synth. Met., 20 (1987), pages 365-371, and DE-A-3325892). It is also known to produce electrically conductive polyaniline powder through oxidative polymerization using chemical oxidizing agents such as persulfate anions (J. C. Chiang et al., Synth. Met. 13 (1986), pages 193 et seq.
From DE-A-3307954 it is known that electrically conductive pyrrole polymers having small particle sizes can be obtained by electrochemical polymerization of pyrrole in the presence of carriers of small size. The carriers contain acidic groups; exemplary of the carriers are sulfonated polystyrene and aluminum oxides.
GB-A-2 134 125 relates to the electropolymerization of pyrrole. A redox species such as potassium ferricyanide can be additionally used.
In various scientific articles it has been suggested to prepare electrically conductive polypyrrole particles by polymerizing pyrrole in an aqueous solution using FeCl.sub.3 as an oxidizing agent in the presence of dissolved methylcellulose, poly(vinyl pyrrolidone) poly(ethylene oxyide) or poly(vinyl-alcohol-co-acetate) which acts as a steric-stabilizer (S. P. Armes et al., J. Chem. Soc., Chem. Commun., 1987, pages 228-290 and R. B. Bjorklund et al., J. Chem. Soc., Chem. Commun., 1986, pages 1293-1295).
DE-A-34 09 462 discloses electrically conductive thermoplastic mixtures of macromolecular compounds and polypyrrole having a small particle size. The macromolecular compounds are polyolefins, styrenic polymers, vinyl chloride polymers, polyamides, polyesters, polyacetales or polyethers. The thermoplastic mixtures are produced by treating a solution of the macromolecular compounds and pyrrole with an oxidizing agent, such as a peroxo acid or a salt thereof.
EP-A-0 229 992 relates to the oxidative polymerization of pyrrole, thiophene, aniline salts etc. with a chemical oxidizing agent such as a perborate, persulfate or percarbonate in the presence of a polymer having anionic surface character. This polymer is dispersed in the reaction medium, for example water, and acts as a polymeric counter-ion for the polymer produced by oxidative polymerization. The polymer having anionic surface character contains strong ionic groups, such as sulfate or sulfonate groups, in the polymer chain. EP-A-0 229 993 relates to the electropolymerization of pyrrole in the presence of a polymer having anionic surface character. This polymer is dispersed in the reaction medium, for example water, and acts as a polymeric counter-ion for the polymer produced by electrochemical oxidation. The same polymers having anionic surface character are mentioned as in EP-A-0 229 92.
EP-A-0 104 726 discloses a polymer composition comprising an electrically conductive polymer associated with a polymeric dopant which stabilizes the polymer in an electrically conductive state. Negatively charged dopants (counter-ions) are used for polypyrrole. These dopants have negatively charged groups in the polymer chain(s). Exemplary thereof are ionizable polysulfonates or polycarboxylates. The electrically conductive polymer is produced by polymerizing a corresponding monomer in the presence of the polymeric dopant and an oxidizing agent. The oxidizing agent may be bonded to the polymeric dopant. The ferric salt of a sulfonated polystyrene is suggested.
Although electrically conductive polymer compositions in powder form are useful in several applications, for example as antistatic fillers for polymers, it is often desirable to produce films or coatings from the electrically conductive polymer compositions. DE-A-3227914 discloses pressing pyrrole polymers at a temperature between 150.degree. C. and 300.degree. C. and a pressure of at least 50 bar; however, this method is rather inconvenient for preparing films or coatings.
Accordingly, others have suggested preparing composite films of polypyrrole and PVC, polyimides, polystyrenes and polymethacrylates (O. Niwa et al., J. Chem. Soc., Chem. Commun., 1984 pages 817 and 818; M.-A. De Paoli et al., J. Chem. Soc., Chem. Commun., 1984 pages 1015 and 1016 and EP-A-191726). In Chemical Abstracts 106 (2):6150c providing an abstract of JP61/123638 it is suggested to soak a PVC film containing aniline with an ammonium persulfate solution. However, these methods are quite complicated and time consuming.
DE-A-3419788 discloses electrically conductive copolymers and blends of polymers which are composed of a polymer component A without a conjugated pi-system and a polymer component B with a conjugated pi-system such as polypyrrole, polythiophene or polyaniline. The disclosed examples of component A are polyvinylchloride (PVC), polybutadiene, polyacrylate, polymethacrylate, copolymers of maleic acid anhydride and styrene, copolymers of butadiene and styrene, chloromethylated polystyrene or polymers which are functionalized with a --NH.sub.2 or --OH group such as poly-(p-aminostyrene) or polyvinylamine which contain redox-active groups which have in the oxidized or reduced state an active potential which is sufficient for oxidation or reduction of component B. As redox-active groups are disclosed complexes or chelates of transition metals, benzoquinone or ferrocene. Unfortunately, the polymer component A has to be functionalized in several steps in order to incorporate the redox-active groups.
U.S. Pat. No. 4,604,427 discloses a method of impregnating a non-porous, swellable or soluble polyamide, polyvinyl chloride, polycarbonate, polyvinyl acetate or polyurethane with pyrrole or aniline and with a chemical oxidant such as ammonium persulfate or iron trichloride. Powders or films are obtained.
U.S. Pat. No. 4,617,353 discloses a process wherein a solution of an electrically non-conducting polymer such as PVC, butadiene copolymer or an olefin-homo- and copolymer is formed in a nonhydrous liquid reaction medium and a pyrrole monomer is contacted in situ with a polymerization initiator selected from the group consisting of anhydrous halides of iron, cobalt or nickel.
R. Yosomiya, Makromol. Chem., Rapid Commun. 7, pages 697 to 701 (1986) discloses that poly(vinyl alcohol), poly(methyl methacrylate) and poly(vinyl chloride) are dissolved in an appropriate solvent together with CuCl.sub.2 or FeCl.sub.3, the solution is cast on a glass plate and dried to form a film which is then reacted with pyrrole.
Unfortunately, the oxidant or polymerization initiator is inhomogenously distributed in the films prepared according to these processes and the mechanical properties of the films are often insufficient.
P. Aldebert et al., J. Chem. Soc., Chem Commun., 1986, pages 1636 to 1638 disclose polymer alloys with mixed electronic and ionic conductivity which have been synthesized from perfluorosulfonated ionomer membranes and monomer precursors of electronically conducting polypyrrole or polyaniline. A commercially available solid acidic Nafion (Trademark) film is soaked in an aqueous solution containing 2M Fe(ClO.sub.4).sub.3 and 0.5 M HClO.sub.4. The proton sites of the Nafion film are exchanged by iron(III) ions. The Nafion film is produced from an ionomeric polymer containing SO.sub.3 group. Polymerization of aniline inside the ionic membrane is obtained by soaking the Fe.sup.3+ exchanged Nafion in a 1M aqueous solution of aniline acidified with H.sub.2 SO.sub.4 or HClO.sub.4.
Due to the interesting properties and the variety of applications of the electrically conductive pyrrole or aniline polymer compositions and due to the limited choice of oxidizing agents and processes which are useful for producing electrically conductive pyrrole or aniline polymers in the form of a film or coating, it remains desirable to provide a new electrically conductive polymer composition comprising polypyrrole, polythiophene, polyaniline or similar polymers which polymer composition can be produced in the form of a film or a coating but also in the form of a powder or granules. Furthermore, it remains desirable to provide a new oxidizing agent and a new process useful for producing such electrically conductive polymer composition.
Surprisingly, it has been found that certain types of polymeric oxidizing agents are useful in the oxidative polymerization of aniline, pyrrole and other aromatic compounds and that the reduced form of the oxidizing agent forms a portion of the electrically conductive polymer composition.